Clara Bow. The original 'It' Girl. Elinor Glyn, the writer who pioneered mass market erotic fiction in the 1920s, described 'It' as — "... that strange magnetism which attracts both sexes... entirely unself-conscious... full of self-confidence... indifferent to the effect..."
Translation for many: Sex appeal.
Yesterday in my rave review of Casino Royale, I mentioned that I thought Daniel Craig had 'It'. Reading Glyn's definition and judging by male and female reaction to the man and his performance, I'm sticking by my call. It has little to do with classic beauty and everything to do with charisma. Daniel Craig has 'It'.
Of course the specifics of 'It' (If you can be specific because really it's undefinable) can be different for different people. Personally, I'd add intelligence and a sense of humor into the mix. What about you? How would you define 'sex appeal'? Who has 'It'?